1. Field of the Invention
The present invention relates to automatic control of electrographic copier and/or printer devices, and more particularly to automatic compensation for changes in the sensitometric characteristics of the image transfer member of such devices.
2. Description of the Prior Art
In electrographic devices such as copiers and/or printers, control of image contrast is desired to produce images having constant and predeterminable quality. The term "contrast" as used herein refers to the rate of change of the output image density D.sub.out with respect to the input image density D.sub.in. Process parameters which determine image contrast include the voltage applied to the image transfer member by the primary charger, the intensity of image exposure, the bias voltage applied to the development station, and the image transfer potential.
Image contrast is also very much affected by the amount of change in transfer member voltage as a function of exposure, known as the "sensitometric characteristic" of the image transfer member. Sensitometric data is sometimes produced by the use of process patches exposed on portions of the transfer member outside image areas thereof. However, such methods have proven to be only partially successful because the data is received from a portion of the transfer member which has not been subjected to the same history of being charged and discharged as has the image areas; and the sensitometric characteristics of the transfer member is greatly influenced by such history.